Mass spectrometry



June 26, 1945. H. W. wAsHBURN MASS SPECTROMETRY Filed Dec. 9, 1943 2 Sheets-Sheet l .Q MmSk 955mb ATTRNEKS June 26, 1945. H. w. wAsHBuRN MASS SPECTROMETRY Filed Dec. 9, 1943 2 Sheets-Sheet 2 INVENTOR. AfA/Pm@ M45/@URN BY f-TTURVEYS Patented June 26, 1945 MASS SPECTROMETRY Harold W. Washburn,

Pasadena, Calif., alsignor to Consolidated Engineering Corporation. Pasadena, Calif.,

a corporation of California Application December 9,\1943, Serial No. 513,525

(Cl. 'I3-18) 16 Claims.

This invention is concerned with mass spectrometry and particularly with the suppression of so-called surface eects" in such instrument.

A mass spectrometer can be employed for both qualitative and quantitative analyses as well as for other purposes, such as the determination of isotope ratios and is, essentially an apparatus for sorting ions according to their mass-to-charge ratio, i. e. their specific mass. In the apparatus, molecules of a mixture to be analyzed are bombarded with ionizing particles such as electrons and are thus converted into ions. The latter are propelled as a beam into an analyzer by means of an electrical potential impressed between spaced electrodes. In the analyzer the beam of unsorted ions is subjected to the influence of a magnetic or electric field and so separated into a plurality of diverging beams of ions having different speciilc masses, each beam being composed of ions of the same specific mass. By shifting the diverging beams relative to an ion collector, they are caused to impinge successively on the collector and are discharged', the current obtained from each beam being an index of the amounts of the ions in each of the beams.

' Ordinarily, the mass spectrometer comprises an ionization chamber maintained under a high vacuum, an electron source such as a heated filament which may be mounted in the chamber or in an adjacent space that communicates therewith. means such as electrodes for impressing a potential on the electrons and shooting them as a beam along a path in the chamber to strike the molecules and ionize them, a plurality of electrodes adapted to impress a potential on the ions thus formed and propel them as a beam into the analyzer, and an ion collector.

Some of the ions do'not follow the prescribed path from the point of their formation in the ionization` chamber to the point of their collection, and instead come in contact with interior surfaces of the apparatus. lIl'ieoretically. by employing conductive surfaces and providing ariequate ground connections the vagrant ions which strike the surfaces can be discharged. In practice, however, I have found that in many cases the charges carried by the vagrant ions are not discharged but accumulate on the interior surfaces and create local electric fields of varying intensity. These fields interfere with the potentials established for propelling the ions through the apparatus and may alter the paths of these ions in a variable and random manner so that the accuracy and reliability of the instrument is seriously affected. I have found further that the vagrant ions have their most serious effect if they are 'deposited on surfaces close to the point of origin of the ions, such for example as a pusher electrode and an outlet electrode which are disposed opposite each other on either side of the path of the electron beam. It is the function of these electrodes to impress on the ions a potential which propels them out of the ionization chamber through an aperture in the outlet electrode, and the accumulation of charges from vagrant lons on these electrodes brings about serious interference with the uniform and proper propulsion of the ion beam.

` Accumulation of static charges from vagrant ions on the interior walls of the ionization chamber or the analyzer chamber also has a deleterious effect upon the accuracy, etc. of the mass spectrum, accumulation on the walls of the ionization chamber being more injurious than that on the analyzer chamber walls.

If a third electrode is employed following the outlet electrode to further accelerate the ion beam, it too will tend to accumulate charges from vagrant ions and these will interfere with the analysis.

I have found that the accumulation of charges from the vagrant ions on metallic surfaces within the mass spectrometer can be prevented by polishing these surfaces highly, the term polishing being employed to describe a rubbing of the surface with 0000 emery polishing paper or the like until the surface acquires a mirror finish. The surfaces may be polished with coarser abrasives and the like prior to the nal polishing. It

is believed that the polishing process renders the surface substantially non-porous and seals any minute pores that may be present. Bumng, i. e. excessive polishing with a bufling wheel wood or the like. is not recommended.

Various conductive metals which :are susceptible to heilig polished to a mirror nish may be employedin the spectrometer, but I have found that chromium alloys (such for example as nichrome and various stainless steels) are most satisfactory. provided that the metal is massive and relatively non-porous. Plated surfaces, even those with a mirror finish, are unsatisfactory, probably because of their relatively high porosity.

ll have found that accumulation of static charges on the polished interior metallic surfaces of a mass spectrometer is minimized by operating the instrument at a relatively high temperature. 'I'hus the ionization chamber should be operated at a temperature in excess of 200 C. and preferably between 200 and 300 C., the optimum in many instances being about 230". C. In consequence, the spectrometer of my invention preferably is provided with means for maintaining the ionization chamber during operation at a 4temperature in excess of 200 C.

I have also found that the effect of the accumulation of static charges on the interior polished surfaces of the spectrometer can be counteracted by employing a relatively high pusher voltage, i. e., by impressing a potential of at least about 1.8 volts between the pusher electrode and the outlet electrode, and that still better results are obtained if a third polished electrode is employed to further accelerate the ion beam passing through the outlet electrode. A high potential, much greater than the pusher voltage and preferably at least about 100 times the pusher voltage, should be impressed between the outlet and third electrodes, optimum results being obtained if the pusher voltage is .5 to 1.0% of that impressed between the outlet and third electrodes, i. e., the voltage between the two outlet electrodes should be between 100 and 200 times the voltage impressed between the pusher electrode and the first outlet electrode. These and other features and advantages of my invention will be more thoroughly understood in the light of the following detailed description taken in. conjunction with the accompanying drawings in which:

Fig. 1 is a sketch of a mass spectrometer assembly, including an ionization chamber assembly or "head, an analyzer and a collector all disposed within an envelope.

Fig. 1A is a plan of the exit plate of the analyzer tube of Fig. 1;

Figs. 2 and 3 are longitudinal sectional views of the head of Fig. 1, taken through the axis of the assembly and, respectively, perpendicular and parallel to a magnetic field impressed on the ionization chamber and analyzer tube by a magnet (not shown)`;

Fig. 4 is a section through the head of Figs. 2 and 3 taken along the line 4--4 of Fig. 3 of the apparatus; Y

Fig. is a section through the filament assembly of Fig. 4 taken along the line 5-5;

Fig. 6 is a section through the filament assembly taken along the line 6 6 of Fig. 5; and

Fig. 1 is a sketch showing the box-shaped shield of they filament assembly.

In the type of mass spectrometer herein described, the envelope and the elements which it encloses are normally mounted in a horizontal plane between the poles of an electromagnet (not shown).

VGeneral assembly in the envelope By reference to Fig. 1, it will be seen that the instrument is provided with a head I0, which includes an ionization chamber, connected at one end to a semicircular analyzer tube II and at the other end to a glass conduit I2 through which a gas sample to be analyzed may be admitted. The head and analyzer tube are enclosed within a glass envelope I3. The end of the envelope at which the head is positioned is enlarged and has a flat section I4 at the very end thereof through which the gas introduction conduit passes. This fiat section is in the form of a glass plate which is secured to the envelope with wax. This wax is kept from melting during the operation of the mass spectrometer by means of a cooling fluid which is circulated through a copper tube I5 in Cir contact with the glass plate adjacent the wax joint.

The enlarged section of the envelope has three side branches. One of these is terminated in a spherical joint I6 which is adapted for connection to a pumping line I1 The second side branch is terminated by an electrical connector I8 having plug terminals for supplying various currents and voltages to electrodes in the ionization chamber. In the third branch there is sealed a lead I9 which may be used as a ground connection. The second and third branches are spaced apart considerably in order to insulate the ground lead from the plug terminals since these are operated at a voltage which may be either positive or negative with respect to ground but which in either event may differ greatly from ground potential.

The end of the envelope remote from the enlarged section has a tapered ground joint surface 20 on the outside. A metal disc 2l acting as an ion collector is mounted on a rod or connector 22 coaxial with the ground joint and within the envelope. This rod extends through a glass seal at the end of the envelope and makes an electrical connection to the grid of an amplifying tube contained in an evacuated metal tube (not shown) which ts over the ground joint.

A heater 30 is provided for heat treating or baking outthe envelope assembly, and comprises the following layers which are wrapped successively about the semi-circular portion of the envelope between the enlarged section and the ground joint:

A layer of asbestos 3|;

A coil of nichrome wire 32 adapted to be heated by electric current;

Two layers of asbestos 33; and

A layer of metal foil 34.

In order to provide against the accumulation of any electrical charges on the inside wall of the envelope, it is coated with a layer 35 of colloidal graphite which is maintained at ground potential through a platinum deposit 36 on the wall and a spiral lead 31 which is connected to the ground lead within the envelope.

The asbestos layers serveto insulate the coil from the foil and also to distribute the heat from the coil over the surface of the glass envelope. The foil aids in heating the assembly by reducing outward radiation of heat from the coil.

The analyzer tube is centered within the glass envelope with the aid of feet 40 extending outwardly from the walls of the analyzer tube and flexible curved leaf springs 4I secured to the outside of the analyzer tube. These centering lsprings are secured at one end rigidly to the analyzer tube and are held in axial alignment with the tube by means of screw heads at the other end which overlap slotted parts of the spring.

Small exhaust ports 42 are drilled into the analyzer tube at numerous points throughout its length at the front and back thereof.

' tial edge of the analyzer tube.

through which ions are projected from the anv alyzer tube onto the collector. e

At the inlet end of the analyzer tube, there is a flange Il to which the head Il is secured b pivoted clamps.

Head, including ionization chamber Details of the head are shown in Figs. 2, 3, 4, 5,

6 and 7. By reference to these figures it will -be seen that the head comprises a block 5l in the form of a thick-walled metal cylinder. a quartz disc 5I secured to the inlet end of the block, a pair of pusher segments I2, 53 separated by a quartz insulator Il and extending into the block bore 55 through the quartz disc, an electron gun 56 (Fig. 3) mounted on the quartz disc, a pair of jaws 51, 50 positioned at the outlet side of the block to define a first slit S1, a second pair of jaws 59. 60 separated from the first pair of jaws by a Pyrex insulating ring 85 and defining a second slit Sz. and a head mounting flange B6 spaced from and electrically connected to the second pair of jaws by means of a copper spacer 61.

The pusher segments are secured to the block by means of a pusher clamp l (which cover a Pyrex pusher locking ring 'H surrounding the two pusher segments) screwed to the block through the quartz disc. 'I'he block and first pair of jaws. the Pyrex insulating ring, the second pa-ir of jaws. the copper spacer, and the head mounting flange are held together by quartz links 12 each of which has an eye at one end (which is positioned over a link-stud screw 13 which extends outwardly from the side of the block wall) and at the other end a second eye perpendicular to the first eye. The second eye is secured to the head mounting flange by means of spring link clips 14.

0n one side of the block, there is a cut-,away section in which the elements of the electron gun are mounted. Electrons from this source are projected through a slotted insert 00 in the wall of the block into the ionization chamber and j thence through a concentric bore 0I in the other side of the block onto an electron catcher 82 which is supported by the quartz disc and ex- "-.tends into a bore 83 drilled part way into the block perpendicular to the bore 8|.

'I'he block, the pusher segments, the central part of the quartz disc and the first pair of slit jaws form a substantially enclosed space that serves as an ionization chamber.

I'he entire assembly is so arranged that the adjacent but spaced edges of the pusher segments, the axis of the electron beam, the axes of slits S1 and Sz, and the axis of the exit slit of the an- `alyzer tube are all parallel to each other and to lines of force of the magnetic field in which the entire envelope assembly is to be placed.

The front faces of the pusher segments. 1. e. the

' races nearest the sut s1 and parallel to the jaws 57|, 5l are highly polished, as are the adjacent faces of the jaws 51, B and the parallel faces of the jaws I9, 60 which define the second slit Sz. Conveniently, the two pusher segments and the jaws 51, 58, 59, 00 are made of alloys such as nichrome containing substantial quantities of chromium. The polishing is conducted by hand with 0000 emery polishing paper or similar fine abrasive or the like until the surface acquires a mirror finish.

Electron beam sources A filament 90 which acts an an electron source in the gun is supported by two L-shaped filament supports 9|, $2 which in turn are fastened to filafr which rest against locking flaps cut into the sides of the filament posts. An apertured electron accelerating electrode $0 is mounted intermediate the filament and the slotted insert or gun barrel 8l through which electrons are to be pro- Delled into the ionization chamber. A boxshaped shield 01 open at opposite ends is disposed around the filament and supported by one of the filament posts with the planes defined by its open ends perpendicular to the axis of the electron A rear filament shield 00 is supported by the other filament post and is disposed in the open end of the box-shaped shield opposite the electrode 98.

The electron accelerating electrode, the boxshaped shield, and the rear filament shield are insulated from each other. Together, they prevent metal evaporated from the filament from striking the quartz disc, for such metal strikes one or the other of the members and condenses before it reaches the quartz insulator.

An external filament shield |00 having a general L-shape is mounted with one flat side in contact with the block at the side remote from the quartz disc, and with the other side of the L generally concentric with the block. This shield thus encloses the electron gun except for an evacuation slot |0l. Thus, the apparatus is provided with an enclosed electron gun chamber |05 which may be evacuated through the slot.

Electrical elements Considered from an electrical standpoint, the block and first jaws represent a first ion accel' erating electrode, and the jaws forming the second slit comprise a second ion accelerating electrode. The two pusher segments are pusher electrodes.

The voltages of the filament, the electron accelerating electrode, the block and the pusher electrodes may be controlled independently. In normal operation, when interested in the analysis of positive ions, these electrodes are all at high positive potentials with respect to the second ion accelerating electrode, which is `at ground.

The rear filament shield is at the same potential as one end of the filament, and the box shield is at the potential of the other end of the filament. The outer filament shield is at the same potential as the block and hence at the same potential as tle first ion accelerating electrode. The outer filament shield and the wall of the block provide an equipotential surface which shields the filament from stray fields that might arise because of any high potential applied between the first ion-accelerating electrode and the second ion accelerating electrode. Any residual stray field that might leak toward the filament because of the presence of' the slot and the non-conductive insulating surface of the quartz disc at the bottom of the electron gun chamber vis prevented from doing so by the presence of the potentials existing on the box-shaped guard disposed around the filament and comprising the electron-accelerating electrode, the open-ended box-shaped.

shield and the rear filament shield.

Operation -trode containing the slit Si.

In the ionization chamber, the molecules of gas are bombarded by an electron beam originating at the filament of the electron gun and passing through the gun barrel 80. An electrical potential is impressed between the pusher electrode (i. e. the interior ends of the two pusher segments) and the jaws 51, 58 which define the slit Si. This potential pushes the ions through the slit S1 as a heterogeneous ion beam. This beam is further accelerated by a potential impressed between the jaws defining the slit S1 and the jaws 59, 60 which dene the slit Sz. 'I'he two -slits act as a collimator and the heterogeneous ion beam or ribbon is projected through the slit S2 into the analyzer tube. In the analyzer tube the ions are acted upon by a magnetic field which separates them according to their specific masses into a plurality of curved homogeneous ion beams which diverge from each other and are focused at different points within the analyzer, for example on the exit plate. By varying the magnetic eld the rad of the curving ion beams may be changed so that the beams are swept successively across the exit slit 41 of the instrument. In this fashion the several beams may be brought to focus successively on the ion collector and there discharged. The currents thus formed constitute the mass spectrum of the gas mixture and, if amplified and recorded, produce a mass spectrogram.

, In a preferred form of my apparatus, means are provided for impressing a substantial potential, say at least about 1.8 volts, between the pusher electrodes and the first accelerating elec- At the same time the apparatus should be so arranged that the pusher potential is as high as possible without causing appreciable defocussing. It should not be less than 1/2% of that impressed between the first and second accelerating electrodes. Optimum operation requires that the voltage between Si and Sz be 100 to 200 times the voltage between Si and the pusher electrodes.

The effect of the polishing with consequent elimination of porosity on the surfaces of the electrodes which propel the ion beam into the analyzer, is to prevent the building up of static charges on these electrodes. I have found that unpolished electrodes tend to produce random errors in the spectrometer.

' As specified above the propelling potential which is impressed between the pusher electrodes and the electrodes 51, 5B (which define the slit Si) is relatively high, and the potential impressed between the slits S1, Sz (i. e. between the ion accelerating electrodes) may be very high-of the order of several hundred volts. These potentials create an electric eld which may effect the emission of electrons by the electron gun, with a serious effect upon the mass spectrum.

The effect of electric elds created by the pro-` pelling potentials in the head of the mass spectrometer is minimized by the metallic shields which are disposed around the electron emitting the quartz plate and this plate is protected by the shields, so that metal vaporlzedand emitted by the filament is deposited on the shields instead of on the insulator.

I have found that a fur-ther increase in uniformity of result with the mass spectrometer can be obtained by painting the outside of the ionization chamber black, thereby increasing the heat radiation from the ionization chamber and decreasing its temperature. The ionization chamber having a black exterior is particularly advantageous when employed with the polished electrodes described hereinbefore.

I claim:

1. In a mass spectrometer having an ionization chamber, means for admitting thereto molecules to be ionized, and means for ionizing the molecules in a restricted region within the chamber by bombarding them with ionizing particles, the combination which comprises a metallic pusher electrode mounted on one side of the region and a metallic outlet electrode mounted on the other side of the region facing the pusher electrode and having an orifice therein extending transverse to the face of the pusher electrode, the adjacent faces of the pusher electrode and the outlet electrode being highly polished, and means for impressing a potential between the electrodes to propel a stream of ions formed in the region through the aperture in the outlet electrode.

2. Apparatus according to claim 1, in which both electrodes are composed of a chromium a1- loy. v

3. In a. mass spectrometer having an ionization chamber, means for admitting thereto molecules to be ionized, and means for ionizing the molecules in a restricted region within the chamber by bombarding them with ionizing particles, the combination which comprises a metallic pusher electrode mounted on one side of the region and a metallic outlet electrode mounted on the other side of the region facing the pusher electrode and having an orifice therein extending transverse to the face of the pusher electrode, the adjacent faces of the pusher electrode and the outlet electrode being highly polished and non-porous, and means for impression a, potential l'between the electrodes to propel a stream of ions formed in the region through the aperture in the outlet electrode.

4. In a mass spectrometer having an ionization chamber, means for admitting thereto molecules to be ionized, and means for ionizing the molecules in a restricted region within the chamber by bombarding them with ionizing particles, the combination which comprises a metallic pusher electrode mounted on one side of the region, a metallic outlet electrode mounted on the other side of the region facing the pusher electrode and having an orifice therein extending transverse to the face of the pusher electrode, and a second metallic outlet electrode mounted beyond the first outlet electrode and in line with it and the pusher electrode, the adjacent faces of the pusher electrode and the first outlet electrode and the face of the second outlet electrode facing the first being highly polished, and means for impressing potentials between the electrodes -to propel a stream of ions formed in the region through the aperture in the outlet electrode.

5. In a mass spectrometer having an ionization chamber, meansfor admitting thereto molecules to be ionized and means for ionizing the molecules in a restricted region .within the chamber by bombarding them with ionizing particles, the

combination which comprises a metallic pusher 'electrode mounted on one side of the region metallic outlet electrode mounted on the other and a side of the region facing the pusher electrode and having an orifice therein extending transverse 6. In a mass spectrometer having an ionizai tion chamber, means for admitting thereto molecules to be ionized, and means for ionizing the molecules in a restricted region within the chamber lby bombarding them with ionizing particles, the combination which comprises a metallic pusher electrode mounted on one side of the region, a rst metallic outlet electrode mounted on the other side of the region facing the pusher electrode and having an orifice therein extending transverse to the' face of the pusher electrode and a second metallic outlet electrode mounted beyond the :firstv outlet electrode and having an orifice therein matching the orifice in the rst outlet electrode, the adjacent faces of the pusher electrode and the first outlet electrode being highly polished, and means for impressing a potential between the pusher electrode and the first outlet electrode to propel a stream of ions formed in the region through the aperture in the outlet electrode.

7. Apparatus according to claim 6 provided with means for impressing between the pusher electrode and the first outlet electrode a potential that is at least of that impressed between the two outlet electrodes.

8. Apparatus according to claim 6 provided with means for impressing between the two outlet electrodes a potential that is between 100 and 200 times that impressed between the pusher electrode and the first outlet electrode.

. 9. Apparatus according to claim provided with an ionization chamber having a black exterior.

10. In a mass spectrometerthe combination trons along a path in the chamber to bombardI the molecules and convert them into ions, a metallic pusher electrode having polished surfaces disposed on one side of the path of the electron beam, a metallic outlet electrode having polished surfaces disposed on the other side of the path of the electron beam facing the pusher electrode with an aperture therein transverse to the surface of the pusher electrode that faces the path of the beam, and means for impressing a potential between the electrodes to propel ions formed in the chamber through the aperture in the outlet electrode.

11. In a mass spectrometer the combination which comprises a metallic ionization chamber the interior surfaces of which are polished, means for admitting molecules to be ionized into the chamber, means for shooting a beam of electrons along a path in the chamber to bombard the molecules and convert them into ions, a metallic pusher electrode having polished surfaces disposed on one side of the path of the electron beam, a metallic outlet electrode having polished surfaces disposed on the other side of the path of the electron beam facing the pusher electrode with an aperture therein transverse to the surface of the pusher electrode that faces the path of the beam, and means for impressing a potential between the electrodes to propel ions formed in the chamber through the aperture inthe outlet electrode.

12. In a mass spectrometer, the combination which `comprises an ionization chamber, means `for admitting molecules to be ionized into the chamber, means for shooting a beam of electrons along a path in the chamber to bombard the mole# cules and convert them into ions, a metallic pusher electrode having polished surfaces disposed on one side of the path of the electron beam, a metallic outlet electrode having polished surfaces disposed on the other side of 'the path of the electron beam facing the pusher electrode with an aperture therein transverse to the surface of the pusher electrode that faces the path of the beam, means for impressing a potential between the electrodes to propel ions formed in the chamber through the aperture in the outlet electrode, and a metallic analyzer chamber communicating with the ionization chamber through the aperture in the outlet electrodes and having polished interior surfaces.

13. In a mass spectrometer the combination which comprises an ionization chamber, means for admitting molecules to be ionized into the chamber, means for shooting a beam of electrons along a path in the chamber to bombard the molecules and convert them into lons, a metallic pusher electrode having polished surfaces disposed on one side of the path of the electron beam,

a metallic outlet electrode having polished surfaces disposed on the other side of the path of the electron beam facing the pusher electrode with an aperture therein transverse to the surface of the pusher electrode that faces the path of the beam, means for impressing a potential between the electrodes to propel lons formed in the chamber through the aperture in the outlet electrode.

and means for maintaining the ionization chamber during operation at a temperature in excess of about 200 C.

14. In a mass spectrometer, the combination which comprises an ionization chamber, means for admitting molecules to be ionized into the chamber, means for shooting a beam of electrons along a path in the chamber to bombard molecules therein and convert them into ions, a metallic pusher electrode having polished surfaces disposed on one side of the path, a metallic outlet electrode disposed in a wall of the chamber on the other side of the beam facing the pusher electrode and having an orifice therein transverse to the path of the electron beam, a metallic third electrode having polished surfaces disposed outside the ionization chamber with an orifice therein in line with the orice in the outlet electrode and with the pusher electrode, means for impressing a potential between the pusher electrode and the outlet electrode for propelling a beam of ions through the orifice in the outlet electrode. and means for impressing between the outlet lelectrode and the third electrode to propel the tallic pusher electrode having polished surfaces disposed on one side of the path, a metallic outlet electrode disposed in a wall of the chamber on the other side of the beam facing the pusher electrode and having an orifice therein transverse to the path of the electron beam, a metallic third electrode having polished surfaces disposed outside the ionization chamber with an orice therein in line with the orice in the outlet electrode and with the pusher electrode, means for impressing a potential of at least 1.8 volts between the pusher electrodey and the outlet electrode for propelling a beam of ions through the orifice in the outlet electrode, and means for impressing between the outlet electrode and the third electrode to propel the ion beam through the orices in the latter, a potential at least about 100 times greater than that impressed between the pusher electrode and the outlet electrode.

16. In a mass spectrometer, the combination which comprises an ionization chamber, means for admitting molecules to be ionized into the fin in line with the oriiice in the outlet electrode and with the pusher electrode, means for impressing a high potential between the outlet electrode and the third electrode to propel ions through the orifice of the latter. and means for impressing between the pusher electrode and the outlet electrode a potential ranging between .5% and 1% of the high potential for propelling ions 20 through the orifice in the outlet electrode.

HAROLD W. WASHBURN. 

